Without question replace the O2 sensor. The muffler won't help with the poor performance and bad gas mileage nearly as much as a fully functioning O2 sensor.

When you get ready for a muffler see if that shop is still open in Prattville across from the Old Larry Puckett Chevy dealership on US31. I used then in the past and they did good custom work for cheap. <img src="/forums/images/graemlins/cheers.gif" alt="" />

PS I'm assuming you're in Montgomery, Al. ??

'86 Toyo 4wd
'04 600RR